Sapphire fiber numerical aperture measurement and sensor fabrication

A sapphire fiber is made of single crystal aluminum oxide with hexagonal crystal structure. It is attractive for high temperature application for its chemical resistance and high melting point of 2053⁰C. It is good for infrared transmission with a small attenuation of 0.13dB per meter at the wavelength of 2.94µm, and it can also deliver very high optical energy. However, a typical sapphire fiber has a rough surface and a nonperfect crystalline structure. Also, a sapphire fiber cannot be grown in a core-clad structure. The un-cladded waveguide imposes a number of undesirable characteristics on sensing applications, e.g., unpredictable light loss and highly multimode operation. Therefore, a suitable cladding layer for sapphire fiber is highly desired for optimizing sapphire fiber optical properties. This thesis aims to investigate the optical properties of single crystal sapphire fibers for development of high performance sensors for various high-temperature sensing applications. The numerical aperture of cladded and uncladded sapphire fibers were measured experimentally and compared for the purpose of selection of proper cladding materials. A novel assembly-free sapphire sensor probe was proposed for temperature and dynamic pressure measurement. The fabricated sensor probe successfully survived high temperatures up to 1560⁰C --Abstract, page iii